Three Vietnamese tourists and an Egyptian guide were killed and 11 others injured when a bomb blast hit their bus on Friday less than four km (2.5 miles) from Egypt's world famous Giza pyramids, authorities said.
The blast is the first deadly attack against foreign tourists in Egypt for more than a year and comes as the tourism sector, a vital source of foreign currency revenue, was recovering from a sharp drop in visitor numbers since the country's 2011 uprising.
No immediate claim of responsibility was reported. Islamist extremists including some linked to Islamic State are active in Egypt and have targeted foreign visitors in the past.
Nine of the injured were Vietnamese tourists, and one was the Egyptian driver, according to official statements.
The interior ministry said the bus was hit by an explosion from an improvised device hidden near a wall on Marioutiya street at around 18:15 (1615 GMT).
About two hours later the vehicle could be seen behind a police cordon with one of its sides badly damaged and the windows blown out, a Reuters reporter said.
Dozens of police, military and firefighters were at the site, on a narrow sidestreet close to the ring road, where traffic was moving normally.
Shortly afterwards, workers brought a pick-up truck to tow the bus away.
An investigator at the scene said the device had likely been planted near the wall.
The injured were taken to the nearby Al Haram hospital, where Prime Minister Mostafa Madbouly told local TV that the guide had died from his injuries.
"The bus deviated from the route secured by the security forces," Madbouly told Extra News TV.
"We have been in contact with the embassy of Vietnam to contain the impact of the incident, and what is important now is to take care of the injured," he said.
The interior ministry confirmed the death of two of the tourists, and the state prosecutors office later said a third had died. In total, 14 Vietnamese tourists had been traveling on the bus, it said.
Egypt's army and police launched a major campaign against militant groups in February, targeting the Sinai Peninsula as well as southern areas and the border with Libya.
The government says fighting Islamist militants is a priority as it works to restore stability after the years of turmoil that followed the "Arab Spring" protests of 2011.
Those events and the bombing of a Russian airliner shortly after it took off from Sharm el Sheikh in 2015 caused tourist numbers to plunge.
The last deadly attack on foreign tourists in Egypt was in July 2017, when two Germans were stabbed to death in the Red Sea resort of Hurghada.